This invention relates generally to ion trap mass spectrometers, and more particularly to mass spectrometers employing an array of miniature ion traps of the same or different sizes, or a combination thereof.
An area of increasing interest in mass spectrometry is that of miniature instrumentation. Recent progress has been made toward the total miniaturization (sample introduction, ion source, mass analyzer, ion detection, data acquisition, and vacuum systems) of all the common types of mass spectrometers. The mass analyzers which are currently the main focus of miniaturization efforts are the linear quadrupole and time-of-flight (TOF) mass analyzers. A number of groups have developed single miniature linear quadrupole analyzers (Syms, R. R. A.; Tate, T. J.; Ahmad, M. M.; Taylor, S. Electron. Lett. 1996, 32, 2094-2095) (Taylor, S.; Tunstall, J. J.; Syms, R. R. A.; Tate, T.; Ahmad, M. M. Electron. Lett. 1998, 34, 546-547) (Syms, R. R. A.; Tate, T. J.; Ahmad, M. M.; Taylor, S. IEEE Trans. Electron Devices, 1998, 45, 2304-2311) (Holkeboer, D. H.; Karandy, T. L.; Currier, F. C.; Frees, L. C.; Ellefson, R. E., J Vac. Sci. Technol. A, 1998, 16, 1157-1162) (Taylor, S.; Tunstall, J. J.; Leck, J. H., Tindall, R. F.; Jullien, J. P.; Batey, J; Syms, R. R. A.; Tate, T; Ahmad, M. M., Vacuum 1999, 53, 203-206) (Freidhoff, C. B.; Young, R. M.; Sriram, S.; Braggins, T. T.; O""Keefe, T. W.; Adam, J. D.; Nathanson, H. C.; Syms, R. R. A.; Tate, T. J.; Ahmad, M. M.; Taylor, S.; Tunstall, J., J. Vac. Sci. Technol. A 1999, 17, 2300-2307).
Arrays of mass analyzers have been used previously, starting with the commercial double-beam Kratos MS30 sector instrument of a generation ago, and, more recently, including multiple linear quadrupoles each of identical size (Ferran, R. J.; Boumsellek, S., J. Vac. Sci. Technol. A 1996, 14, 1258-1265) (Orient, O. J.; Chutjian, A.; Garkanian, V., Rev. Sci. Instrum. 1997, 68, 1393-1397). In the latter cases, multiple analyzers are specifically used in order to provide higher ion currents while maintaining the favorable operating conditions of physically smaller devices, including higher pressure tolerance and lower working voltages. As an example of this approach, Kirchner (Kirchner, N. J.: U.S. Pat. No. 5,206,506, 1993) proposed a parallel electrostatic ion processing device composed of a parallel series of channels. Each channel was designed to store, process, and then detect ions. Due to the parallel architecture, high ion throughput and high capacity were expected.
Miniature mass spectrometers that can be operated in non-laboratory and harsh environments are of interest for continuous on-line and other monitoring tasks. Simplicity of operation and small size are the premier qualities sought in these devices. Only modest performance in terms of resolution and dynamic range is needed to address many of the problems to which these small instruments might be applied. Miniaturization of the mass analyzer must be accompanied by miniaturization of the entire system, including the vacuum system and control electronics. The ion trap mass analyzer is physically small. Nearly a decade ago a miniature version (2.5 mm internal radius) was described by Kaiser et al. (Kaiser, R. E.; Cooks, R. G.; Stafford, G. C.; Syka, J. E. P.; Hemberger, P. H., Int. J. Mass Spectrom. Ion Processes 1991, 106, 79-115).
One major issue with miniaturized mass spectrometers is the pressure tolerance of the device. Currently, pumping systems are size, power and weight prohibitive, and the miniature devices available do not provide the pumping speeds or base pressures associated with full-size pumps. Offsetting this is the fact that the pressure tolerance of small analyzers is greater than that of larger analyzers, since the shorter path lengths decrease the probability of ion/neutral atom or molecule collisions. Even though ion traps have relatively long path lengths, collisions with gases of lower mass and higher ionization potential have beneficial effects on resolution since they cool ions to near the center of the device (Stafford G. C.; Kelley, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F., Int. J. Mass Spectrom. Ion Processes 1984, 60, 85-98). The result is that quadrupole ion traps are the most pressure-tolerant of all the major types of mass analyzers, and small ion traps should be even more so. A pressure tolerant analyzer like the quadrupole ion trap, therefore, is of special interest as a miniature mass spectrometer, since base pressure can be higher and pumping capacity lower, allowing use of a simpler pumping system.
In the search for a robust mass analyzer for miniaturization, the quadrupole ion trap is a prime candidate due to its overall performance characteristics and operating conditions that are beneficial for the miniaturization process. Operation of the trap using simplified-applied voltages simplifies the control electronics needed to operate the ion trap as a mass analyzer. Also, given that a reduction in size causes a reduction in ion trapping capacity, a method to gain back total ion trapping capacity is needed when miniaturized ion traps are used, and the use of multiple individual traps is suggested for this purpose.
The conventional method of operating a hyperbolic quadrupole ion trap as a mass spectrometer is to perform a mass-selective instability scan. In this experiment the amplitude of the applied rf voltage is scanned so as to force ions of increasing m/z ratios into unstable trajectories, causing them to leave the trap and allowing them to impinge on an external detector such as an electron multiplier (Stafford G. C.; Kelley, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F., Int. J. Mass Spectrom. Ion Processes 1984, 60, 85-98). The relationship between the parameters involved is given by the Mathieu equations. The solution for ion motion in the z (axial) direction can be expressed in terms of the Mathieu parameter qz where:       q    z    =                    8        ⁢                  xe2x80x83                ⁢        z        ⁢                  xe2x80x83                ⁢        V                    m        ⁢                  xe2x80x83                ⁢                              Ω            2                    ⁢                      (                                          r                0                2                            +                              2                ⁢                                  xe2x80x83                                ⁢                                  z                  0                  2                                                      )                                .  
In this equation, V is the amplitude of the trapping rf voltage, m is the mass of the ion of interest, r0 and z0 are the inscribed dimensions of the ion trap, and xcexa9 is the angular frequency of the rf voltage. It has been previously noted that, in principle, at a fixed value of qz, variation in V, xcexa9 or r will correspond to selection of ions of different m/z values (Kaiser, R. E.; Cooks, R. G.; Stafford, G. C.; Syka, J. E. P.; Hemberger, P. H. Int. J. Mass Spectrom. Ion Processes 1991, 106, 79-115) (Kaiser, R. E.; Cooks, R. G.; Moss, J.; Hemberger, P. H. Rapid Comm. Mass Spectrom. 1989, 3, 50-53). Indeed, scans of V have been used to record mass spectra, the value of qz being fixed by the boundary for ion stability or some other operating point in the stability diagram (Stafford, G. C.; Kelley, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F. J. Int. J. Mass Spectrom. Ion Processes 1984, 60, 85-98).
A cylindrical ion trap (CIT) was first described by Langmuir for use as an ion containment device, but not as a mass spectrometer. Subsequently, the use of CITs has focused mainly on ion storage, although recent experiments by Badman (Badman, E. R.; Johnson, R. C.; Plass, W. R.; Cooks, R. G. Anal. Chem. 1998, 70, 4896-4901) and Kornienko (Kornienko, O.; Reilly, P. T. A.; Whitten, W. B.; Ramsey, J. M. Rapid Commun. Mass Spectrom. 1999, 13, 50-53) (Kornienko, O.; Reilly, P. T. A.; Whitten, W. B.; Ramsey, J. M. Rev. Sci. Instrum. 1999, 70, 3907-3909) have shown them to perform well as mass spectrometers. CITs are also simpler to machine than standard hyperbolic quadrupole ion traps, especially on the millimeter scale. A cylindrical ion trap (CIT) consists of a barrel-shaped central ring electrode with two flat endcap electrodes, and as such, it is extremely simple to machine compared to the hyperboloid shapes of the electrodes in the standard quadrupole ion trap.
It is an object of the present invention to provide a mass spectrometer consisting of an array of quadrupole ion traps each element of which is operated using the same rf and dc trapping signals.
It is another object of the present invention to provide a mass spectrometer having simple miniaturized control electronics and pumping systems.
There is provided a mass spectrometer in which, in the first embodiment, each element of an array is an ion trap whose dimensions are proportionately varied. This allows the size (r0 and z0) of the device to be used as a variable in the Mathieu stability equation to trap ions of different mass/charge ratios in the individual ion traps with the same rf and dc trapping voltages. Each trap operates in the mass selective stability mode to trap ions of a given m/z value or range of m/z values. Isolation of ions in a quadrupole ion trap is commonly achieved by applying, along with the trapping rf voltage, a dc voltage between the ring electrode and the endcap electrodes or, alternatively, by the use of a waveform applied to one or more electrodes to resonantly eject ions of one or multiple mass/charge ratios through use of a pulse with frequency components equal to the frequencies of motion of the ions to be ejected. In this invention, the mass range selected for isolation is controlled via the applied voltages to be for a single m/z value (as is typically done) to a wide range of masses, including the entire mass range.
In the second embodiment, the array consists of identical-sized ion traps, also operated under common conditions. This type of array can be operated in a similar manner as the first embodiment, using same methods of ion isolation, ejection and detection. In this case, the invention allows increased ion trapping capacity over a single-sized ion trap operated under identical conditions, which improves overall signal intensity. Alternatively, with appropriate methods of ionization and injection, it allows simultaneous analysis of multiple samples using the same array mass spectrometer and the same vacuum, electronics and data systems.
In a further embodiment, the arrays may be operated in series whereby the first array can be used to accumulate ions before they are injected into the second array.